Seregi Skripal, 66, and his daughter, Yulia Skripal, 33, are in critical condition, and while authorities have given few details of what happened to them, their case is a chilling reminder of a slew of mysterious Russia-linked deaths on British soil over the last decade.

A British inquiry found that two Russian agents poisoned Alexander Litvinenko at a London hotel bar in 2006 by spiking his green tea with the highly radioactive polonium-210. Litvinenko died a slow and painful death over weeks after the poisoning and always insisted that Russian President Vladimir Putin and the Kremlin were responsible for what happened to him.

In a statement from his deathbed, he said: “You may succeed in silencing one man, but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life.”

Officials have always dismissed the accusation as “nonsense,” but suspicions linger.

The inquiry, led by judge Robert Owen, said that Putin “probably approved” the ex-spy’s killing.

The Kremlin has always denied the accusation, as did the two agents accused of the poisoning, whom the Russian government refuses to extradite to Britain.

Litvinenko had worked for the FSB, Russia’s successor agency to the KGB, the former Soviet secret police and intelligence agency. He specialized in tackling organized crime and his last job at the agency was heading up its anti-corruption department, after the collapse of the Soviet Union, a position that made him many enemies.

After leaving the FSB, Litvinenko blamed the service for orchestrating a series of apartment bombings in Russia in 1999 that left hundreds dead and led to Russia’s invasion of Chechnya later that year.

He went to the UK in 2000 after turning whistle-blower on the agency. According to his widow, Marina Litvinenko, he then started working for Britain’s security services.